Previous work from this laboratory has shown that about half the ribosomal RNA (rRNA) genes in Drosophila are interrupted, and that these interrupted genes are inactive, i.e., they are pseudogenes. By studying run-off transcripts in isolated nuclei we obtained indications that the interrupted rRNA genes are transcribed but terminate at or close to the site of interruption, yielding incomplete RNA molecules. This result begins to provide information about the mode of inactivation of pseudogenes. To establish a system for the rapid assay of transcriptional capability of any segment of Drosophila DNA in vivo a method was adapted for DNA-mediated gene transfer into two Drosophila cell lines. The marker gene chloramphenicol acetyl transferase could be expressed in these cells when placed under the control of promoters from the heat shock protein 70 gene or the element copia. This assay method is being used currently to study the transcriptional activity of interrupted and uninterrupted rRNA genes after in vitro modification and reintroduction into the cell. The study of DNA sequences which interrupt rRNA genes has led to the discovery of a novel class of transposable DNA sequences in Drosophila. Detailed analysis in the current year has shown that these sequence elements carry an oligo(A) stretch at one end but lack the long terminal repeats commonly found at the ends of many transposons. The maternal effect developmental gene fs(1)h is being studied in collaboration with Drs. Gans and Fourquignon in Gif. This gene is known to lead to homeotic transformations under certain conditions, and provides a rare example of a maternal effect gene apparently involved in the specification of body plan. To study the molecular nature of this gene we aim to isolate it by the method of chromosomal walking. So far, about 85 kb of DNA have been isolated from the relevant region, and various mutants and rearrangement stocks are being mapped in relation to the DNA.